This invention relates to driver circuits for controlling the response of a diode laser to digital input signals.
Optical recording involves the use of a ight source such as a laser diode to "write" information on an optical disk, usually by locally altering the disk surface to form a spot or hole which subsequently can be sensed by a reduced power laser in "reading" the information placed on the disk. As is the case with magnetic recording and other information processing techniques, the ever-present goal in optical recording is to increase the speed at which data can be written and read. A significant challenge encountered in optical data storage is the need for an effective means to force a laser diode to rapidly respond to incoming digital data.
One approach to this problem is shown in U.S. Pat. No. 4,027,179 to Kawamoto et al. granted May 31, 1977. A carrier-injected avalanche device 10 causes an injection laser 22 to emit light responsive to signals from a pulse generator 28. An offset voltage is provided by a capacitor 37 through a resistor 32, to rapidly deplete any electric field caused by accumulation of electrons and holes at a control junction 19 between semiconductor layers 14 and 12 of the avalanche device. This enables a higher pulse repetition rate in the injection laser. In U.S. Pat. No. 4,295,226 to Dombrowski granted Oct. 13, 1981, the junction depletion layer of a light emitting diode 13 is depleted when a transistor 16 is enabled, thus to provide a positive current overdrive to the LED.
The Dombrowski patent further teaches a non-zero "off" state for current to the LED. Reduction of the current to zero then provides a negative overdrive effect without actually reverse biasing the diode. Pre-biasing of a field effect transistor is shown in U.S. Pat. No. 4,243,951 to Wolkstein et al. granted Jan. 6, 1981. The pre-biasing produces a current in FET 17 just below the "lasing" level of a laser diode 26. Such pre-biasing reduces the time for diode 26 to respond to the high-going pulse of a digital signal 12.
It is an object of the present invention to provide a simple, low-cost means for rapidly activating and deactivating a laser diode in response to a digital signal. It is a further object of the invention to achieve such response in a laser diode through a coaxial cable with a minimum loss of speed even at high current requirements with minimum impact on adjacent circuitry. Yet another object of the invention is to produce rapid response in a laser diode without prebiasing it and without a separate circuit for depleting a junction between semiconductor layers.